The present invention generally relates to dispensing liquids onto a surface. More particularly, the present invention relates to methods of spin dispensing process liquids, such as photoresist, onto a surface of a semiconductor wafer that more evenly distribute the process liquids over the surface and reduce the number of defects resulting from maldistribution of the liquid on the surface.
Integrated circuits are solid state devices in which electrical components and electrical connections between the components are incorporated into a solid matrix. The circuits are formed by the strategic placement of various conducting, semiconducting and insulating materials on a substrate. The development of the integrated circuit has led to the miniaturization of electronics by providing a strong matrix to support and protect fragile miniaturized components and connections and by facilitating the placement of the electrical components in close proximity. The integrated circuit has further served to increase the reliability of electronic devices by the elimination of moving parts and fragile electrical wiring and connections.
Integrated circuits are typically constructed by depositing a series of individual layers of predetermined materials on a wafer shaped semiconductor substrate, or "wafer". The individual layers of the integrated circuit are in turn produced by a series of manufacturing steps. For example, silicon dioxide is typically deposited over a previously formed circuit layer to provide an insulating layer for the circuit. Subsequent circuit layers are formed on the wafer using a radiation alterable material, known as photoresist.
Photoresist materials are generally composed of a mixture of organic resins, sensitizers and solvents. Sensitizers are compounds, such as diazonaphthaquinones, that undergo a chemical change upon exposure to radiant energy, such as visible and ultraviolet light. The irradiated sensitizer generally has different physical properties than the non-irradiated sensitizer, such as differing solvation characteristics. Resins are used to provide mechanical strength to the photoresist. Solvents are added to lower the viscosity of the photoresist, which facilitates a more uniform application of the liquid over the surface of the wafers.
A smooth, level layer of photoresist is formed on the wafer to provide a proper surface for depositing additional layers in the production of the circuit. After the photoresist layer is formed, it is typically heated to evaporate the solvents and harden the layer.
The hardened photoresist layer is then selectively irradiated to produce a layer having varying solvation characteristics. A radiation mask having transparent and opaque portions that define the next circuit layer pattern is used in conjunction with a radiation source to selectively expose the layer to radiation.
Following irradiation, the photoresist layer is exposed to a chemical, known as developer, in which either the irradiated or the nonirradiated photoresist is soluble. The soluble portion of the photoresist is dissolved exposing portions of the underlying insulating layer in the pattern defined by the mask. Developer solutions need to be uniformly distributed over the substrate space to facilitate uniform dissolution of the photoresist layer.
Photoresist and developer solutions and other process liquids are typically applied to the wafer while the wafer is being spun on a rotating chuck, using a technique known as a spin dispensing, or coating. The liquid may be dispensed before the wafer is spun (i.e., statically) or while the wafer is spinning (i.e., dynamically). The spinning of the wafer distributes the liquid over the surface of the material.
The final thickness and uniformity of a process liquid layer depends on a number of variables. Spin variable, such as spinning speed, time and acceleration, dispense techniques, times and quantities can greatly affect the layer. The system pressure, temperature, and exhaust flow rate, as well as the physical properties of the process liquid, such as the viscosity and the vapor pressure of the volatile components, also affect the thickness and uniformity. If the spin variables are not matched to the process liquid, the resulting layer may contain an unacceptable number of defects. Commonly, a string of defects occurs starting near the center of the surface and extending radially perpendicular to the edge of the surface. The strings of defects, called striations, are thought to be air bubbles trapped in the liquid during the coating process.
Generally, efforts to provide a more uniform layer of process liquid during spin dispensing have focused on either changing the dispense nozzle design to provide a different dispense pattern and varying the spin pattern of the substrate to alter the distribution pattern of the liquid after it has been dispensed onto the surface of the substrate. Examples of the different nozzles used to dispense process liquid can be found in U.S. Pat. No. 5,002,008 issued to Ushijima et al., U.S. Pat. No. 5,020,200 issued to Mimasaka et al. and U.S. Pat. No. 5,429,912 issued to Neoh to name a few.
A number of spin techniques have been developed attempting to uniformly distribute the process liquid over the surface of the substrate. For example, U.S. Pat. No. 4,741,926 issued to White discloses spin coating an organic material at a speed of not less than 4000 rpm, preferably 6000-8000 rpm, until a build up of coating is detectable on a side wall of a topographical feature on the surface. The rotational speed is then decelerated to less than 4000 rpm, preferably 1000-3500 rpm to produce a layer of desired thickness. A difficulty with the White process is that a sensor must be positioned to detect the build up of coating material on the side wall. Because the location of the build up will most likely vary from wafer to wafer, it may be difficult to reliably or consistently detect. Also, an excess amount of coating material may be necessary to compensate for material spun off the wafer before the build up of coating material is detected on the side wall.
In U.S. Pat. No. 5,405,813 issued to Rodrigues, methods are disclosed involving four different spinning steps for optimizing the distribution of photoresist on a semiconductor wafer. The methods provide for spinning the wafer at a first rotational speed and then decelerating the wafer. Photoresist is applied during the deceleration until a second rotational speed is reached and the dispensing of the photoresist is stopped. The wafer is then accelerated to a third rotational speed to produce a layer of a desired thickness and further accelerated to a fourth rotational speed to dry the coating layer.
In U.S. Pat. No. 5,453,406 (the '406 patent) issued to Chen, methods are disclosed for providing an aspect ratio independent spin on glass (SOG) coating by dispensing a first layer of the coating at a low speed followed by second layer of the coating dispensed at a high speed. This method requires that one additional step be added to the production process for each coating step in the prior art processes.
U.S. Pat. No. 5,567,660 issued to Chen et al. discloses a process similar to the '406 patent. A SOG is statically dispensed on the substrate and the substrate is spun at a first low speed followed by a second higher speed. A difficulty encountered in this method, as with other methods employing lower spin speeds, is that radial striations may occur in the coating layer. Radial striations are generally a result of either the dispense liquid not having sufficient momentum to be distributed evenly over a nonplanar surface or the liquid not preferentially wetting the substrate surface.
U.S. Pat. No. 5,066,616 issued to Gordon discloses methods for producing a layer of photoresist. The Gordon methods involve dispensing a liquid solvent underlayer to initially wet the surface of the wafer. A photoresist layer is then spun on top of the solvent layer as a means to better ensure more complete coverage of the wafer surface by the photoresist. The Gordon methods provide a solution of the problem of mottling; however, the methods require the additional dispensing of a solvent layer prior to the photoresist layer. The solvent layer may increase the possibility of defect formation following the evaporation of the solvent underlayer and nonuniform thinning of the photoresist by the solvent underlayer.
Thus, it is apparent that a need exists for an improved method for spin dispensing process liquids, such as photoresist, which overcomes, among others, the above-discussed problems to produce a more uniform layer of process liquid over the surface of the wafer.